A much lower redox potential than that of the mixed Sec,Cys‐peptide and particularly of the related Cys,Cys‐peptide is a feature of the cyclic selenocystine‐peptide (Sec,Sec‐peptide). These findings that were obtained with appropriately modified glutaredoxin‐octapeptides at pH 7 [Eq. (1); X = S, Se; DTT = dithiothreitol], open interesting new approaches for the design of productive intermediates in the oxidative folding of synthetic peptides and recombinant proteins. Moreover, such seleno derivatives may represent useful heavy metal analogs for X‐ray structure analysis.

The methodology of regioselective cysteine pairings in synthetic multiple‐cystine peptides has progressed in the past years to an efficiency that allows for at least three specific inter‐ and intrachain disulfide bridgings. Conformational studies on various multiple‐cystine peptides like hormones, protease inhibitors, and toxins revealed that these bioactive peptides, generated by posttranslational processing of precursor proteins, are folded into miniprotein‐like compact globular structures of remarkable stability. This strongly suggests protein domain or subdomain properties of these families of peptides, and thus sufficient sequence‐encoded information for correct oxidative refolding under appropriate experimental conditions. From intensive research on the mechanisms and pathways of oxidative refolding of proteins in vivo and in vitro, the efficient methods have emerged for simulating nature in the regeneration of native folds not only for intact proteins, but also for protein domains and subdomains. In fact, the results obtained in the oxidative folding of excised protein fragments and of relatively low mass products of posttranslational processings show that this procedure is indeed a simple way of preparing peptides with several disulfide bonds, if optimization of reaction conditions is performed in terms of redox buffer, temperature, and additives capable of disrupting aggregates and of stabilizing nascent secondary structures. Moreover, with increased knowledge about stable, small natural cystine frameworks, their use instead of artificial templates should facilitate engineering of synthetic miniproteins with specific conformation and tailored functions. © 1996 John Wiley & Sons, Inc.

Using the Fmoc/tBu protection scheme and the p-methoxybenzyl derivative of selenocysteine, the synthesis of related peptides in the selenol-protected form could be optimized by operating the coupling steps in the absence of auxiliary bases and by reducing the piperidine treatment to the minimum time required for quantitative Fmoc cleavage. Under these conditions, beta-elimination of the p-methoxybenzylselenol as the main side reaction of these syntheses, as well as epimerization of the protected selenocysteine, was largely suppressed. Conversion of the selenol- and thiol-protected bis-selenocysteine and selenocysteine, cysteine peptides into the related cyclic monomeric forms by iodine-mediated oxidation failed since a complex mixture of compounds was produced. Cleavage of the selenoether bond with mercuric acetate was found to proceed smoothly, but displacement of the heavy metal ions by treatment with excesses of thiols or hydrogen sulphide was unsuccessful since a stable Hg2+ diselenide complex was obtained. However, oxidation was achieved in good yields by the dimethylsulphoxide/trifluoroacetic acid procedure and the peptides were then used for determining the redox potential of the diselenide and selenide/sulphide bridge, respectively.

Using the Fmoc/tBu protection scheme and the p-methoxybenzyl derivative of selenocysteine, the synthesis of related peptides in the selenol-protected form could be optimized by operating the coupling steps in the absence of auxiliary bases and by reducing the piperidine treatment to the minimum time required for quantitative Fmoc cleavage. Under these conditions, beta-elimination of the p-methoxybenzylselenol as the main side reaction of these syntheses, as well as epimerization of the protected selenocysteine, was largely suppressed. Conversion of the selenol- and thiol-protected bis-selenocysteine and selenocysteine, cysteine peptides into the related cyclic monomeric forms by iodine-mediated oxidation failed since a complex mixture of compounds was produced. Cleavage of the selenoether bond with mercuric acetate was found to proceed smoothly, but displacement of the heavy metal ions by treatment with excesses of thiols or hydrogen sulphide was unsuccessful since a stable Hg2+ diselenide complex was obtained. However, oxidation was achieved in good yields by the dimethylsulphoxide/trifluoroacetic acid procedure and the peptides were then used for determining the redox potential of the diselenide and selenide/sulphide bridge, respectively.

Selenocystein (Sec) kann, bezogen auf die ribosomenvermittelte Proteinsynthese, als die 21. Aminosaure betrachtet werden.", 21 Es kommt in vielen prokaryontischen und eukaryontischen Proteinen vor, wobei die Natur das UGA-Stopcodon zum Einfugen dieser Selenoaminosaure verwendet statt eines posttranslationalen Modifikationsmechanismus. Die meisten der bislang identifizierten Selenoproteine sind Redoxenzyme, die wegen des niedrigen pK,-Werts der Selenolfunktion (5.73 fur Sec, 8.53 [lo-171 Fragmentes in der oxidierten Form zeigt ein intensives Signal bei 6 = 266.6, wahrend in Gegenwart von DTT in groBem UberschuB (103fach)